In a typical consumable electrode arc welding, welding is performed by feeding a welding wire as a consumable electrode at a constant feeding rate and generating an arc between the welding wire and base material. In the consumable electrode arc welding, both the welding wire and the base material are mostly placed in a welding state in which a short-circuiting period and an arc period are alternately repeated.
In order to further improve welding quality, there has been proposed a welding method of alternating feeding of the welding wire between a forward feeding and a reverse feeding periodically (see Patent Document 1, for example).
In this welding method, the feeding of the welding wire is required to be changed between the forward feeding state and the reverse feeding state at a high speed, e.g., every about 5 ms. To this end, a feeding motor is installed near a tip of a welding torch so as to shorten a feeding path from the feeding motor to the tip of the welding torch. However, as only the feeding motor of a small size is permitted to be installed near the tip of the welding torch due to a limited installation space, a feeding torque becomes insufficient sometimes. To solve this problem, a push-pull feeding control system is configured to use two feeding motors in a manner that one (push side) of the feeding motors is installed at an upstream side of the feeding path and the other feeding motor (pull side) is installed neat the tip of the welding torch on the downstream side of the feeding path. In this push-pull feeding control system, the push-side feeding motor is controlled to perform a constant feeding-rate operation in the forward feeding state, whilst the pull-side feeding motor is controlled to alternate the forward feeding and the reverse feeding periodically. Hereinafter this welding method will be explained.
FIG. 3 is a waveform diagram of the welding method in which the push-pull feeding control system is employed to periodically repeat the forward feeding and the reverse feeding as to the feeding rate. (A) of this figure shows individual waveforms of a pull feeding-rate setting signal Fr (solid line) and a pull feeding rate Fw (broken line), (B) of this figure shows a waveform of a welding current Iw, (C) of this figure shows a waveform of a welding voltage Vw, and (D) of this figure shows a waveform of a push feeding rate Pw. Hereinafter explanation will be made with reference to this figure.
As shown in (A) of this figure, in each of the pull feeding-rate setting signal Fr and the pull feeding rate Fw, an upper side than 0 represents a forward feeding period and a lower side represents a reverse feeding period. The forward feeding represents feeding of the welding wire in a direction approaching the base material, whilst the reverse feeding represents feeding of the welding wire in a direction separating from the base material. The pull feeding-rate setting signal Fr has a waveform which changes sinusoidally and shifts on the forward feeding side. The tip of the welding wire is fed forwardly and reversely at the pull feeding rate Fw. As an average value of the pull feeding-rate setting signal Fr is positive, the welding wire is fed forwardly in average. As shown in (D) of this figure, the push feeding rate Pw is controlled to perform a constant feeding-rate operation in the forward feeding state based on a push feeding-rate setting signal (not shown) set in advance. The average value of the pull feeding-rate setting signal Fr and the push feeding-rate setting signal are set to be equal to each other.
As shown by the solid line in (A) of this figure, the pull feeding-rate setting signal Fr is 0 at a time point t1. A period from the time point t1 to a time point t2 corresponds to a forward feeding acceleration period. The pull feeding-rate setting signal Fr is the maximum value of the forward feeding at the time point t2. A period from the time point t2 to a time point t3 corresponds to a forward feeding deceleration period. The pull feeding-rate setting signal Fr is 0 at the time point t3. A period from the time point t3 to a time point t4 corresponds to a reverse feeding acceleration period. The pull feeding-rate setting signal Fr is the maximum value of the reverse feeding at the time point t4. A period from the time point t4 to a time point t5 corresponds to a reverse feeding deceleration period. Then a period from the time point t5 to a time point t6 is the forward feeding acceleration period again, and a period from the time point t6 to a time point t7 is the forward feeding deceleration period again. For example, the maximum value of the forward feeding is 50 m/min, the maximum value of the reverse feeding is −40 m/min, the forward feeding period is 5.4 ms, and the reverse feeding period is 4.6 ms. In this case, the single period is 10 ms, and the short-circuiting period and the arc period are alternately repeated with 100 Hz. An average value of the pull feeding rate Fw in this case is about 4 m/min (an average value of the welding current is about 150 A).
As shown by the broken line in (A) of this figure, the pull feeding rate Fw is an actual feeding rate. The pull feeding rate represents a sinusoidal wave which rises and falls later than the pull feeding-rate setting signal Fr. The pull feeding rate Fw is 0 at a time point t11. A period from the time point t1 to a time point t21 corresponds to a forward feeding acceleration period. The pull feeding rate Fw is the maximum value of the forward feeding at the time point t21. A period from the time point t21 to a time point t31 corresponds to a forward feeding deceleration period. The pull feeding rate Fw is 0 at the time point t31. A period from the time point t31 to a time point t41 corresponds to a reverse feeding acceleration period. The pull feeding rate Fw is the maximum value of the reverse feeding at the time point t41. A period from the time point t41 to a time point t51 corresponds to a reverse feeding deceleration period. Then a period from the time point t51 to a time point t61 is the forward feeding acceleration period again, and a period from the time point t61 to a time point t71 is the forward feeding deceleration period again. This is due to transient characteristics of the pull feeding motor and a feeding resistance of the feeding path.
A welding power supply controlled to a constant voltage is used for the consumable electrode arc welding. Short circuit between the welding wire and the base material occurs mostly before or after the maximum value of the pull feeding rate Fw in the forward feeding at the time point t21. This figure shows a case where the short circuit occurs at a time point t22 during the forward feeding deceleration period after the maximum value in the forward feeding. If the short circuit occurs at the time point t22, the welding voltage Vw rapidly reduces to a short circuit voltage value of a few volts as shown in (C) of this figure, whilst the welding current Iw increases gradually as shown in (B) of this figure.
As shown in (A) of this figure, from the time point t31, as the pull feeding rate Fw is placed in the reverse feeding period, the welding wire is reversely fed. The short circuit is released by this reverse feeding, and hence an arc is generated again at a time point t32. The arc is regenerated mostly before or after the maximum value of the pull feeding rate Fw in the reverse feeding at the time point t41. This figure shows a case where the arc is generated at the time point t32 during the reverse feeding acceleration period before the maximum value of the reverse feeding. Thus a time period from the time point t22 to the time point t32 becomes the short-circuiting period.
If the arc is regenerated at the time point t32, the welding voltage Vw increases rapidly to an arc voltage value of several tens of volts as shown in (C) of this figure. As shown in (B) of this figure, the welding current Iw starts changing from the maximum value state during the short-circuiting period.
As shown in (A) of this figure, during a period from the time point t32 to the time point t51, as the pull feeding rate Fw is in the reverse feeding state, the welding wire is raised and hence a length of the arc becomes longer gradually. If the arc length becomes longer, the welding voltage Vw increases, and hence the welding current Iw reduces due to the constant voltage control. Thus during the reverse feeding period in the arc period from the time point t32 to the time point t51, the welding voltage Vw increases gradually as shown in (C) of this figure, whilst the welding current Iw reduces gradually as shown in (B) of this figure.
Then the next short circuit occurs at a time point t62 within the forward feeding deceleration period of the pull feeding rate Fw from the time point t61 to the time point t71. A time period from the time point t32 to the time point t62 corresponds to the arc period. As shown in (A) of this figure, during a period from the time point t51 to the time point t62, as the pull feeding rate Fw is in the forward feeding state, the welding wire is forwardly fed and hence a length of the arc becomes shorter gradually. If the arc length becomes shorter, the welding voltage Vw reduces, and hence the welding current Iw increases due to the constant voltage control. Thus during the forward feeding period in the arc period from the time point t51 to the time point t62, the welding voltage Vw reduces gradually as shown in (C) of this figure, whilst the welding current Iw increases gradually as shown in (B) of this figure.
As described above, in the welding method of repeating the forward feeding and the reverse feeding of the welding wire alternately, the repetition period of the short circuit and the arc can be set to a desired value despite that such the setting is impossible in the related art of the feeding at a constant feeding rate. Thus a generation ratio of spatter can be reduced and improvement of welding quality such as improvement of bead appearance can be achieved.